Radio communication channels such as HF, VHF and UHF introduce distortion in the form of multipath and fading into the originally transmitted signal. A result of these types of channels is inter-symbol interference (ISI), which occurs if the modulation bandwidth exceeds the coherent bandwidth of the radio channel and causes the modulation pulses to spread in time to adjacent symbols. Intersymbol interference can also be caused by the radio channel exhibiting time and frequency dispersion (e.g., delay spread and Doppler spread) due to the presence of signal reflectors/scatterers in the environment or the relative motion of transmitter and receiver. Intersymbol interference has also been known to cause bit errors at the receiver, which distorts the intended message content. To address such transmission channel distortion, many different types of channel estimation algorithms and adaptive equalizers have been included in the receivers.
Modern communication systems are requiring wider and wider bandwidth signals in order to support the data rates desired by users. The large amount of legacy equipment in the HF/UHF/VHF bands can at times cause unintentional interference to these new wideband systems. In addition, intentional jamming can also occur. Adaptive filters are commonly used in communication systems to reduce the effects of narrowband interferers. An adaptive filter can process the communication signal prior to acquisition, demodulation, equalization and decoding. Adaptive filters do not require knowledge of the channel in advance and incorporate an Infinite Impulse Response (IIR) or Finite Impulse Response (FIR) filter with adaptive filter coefficients that adjust themselves to achieve a desired result, such as minimizing unwanted narrowband interference of the input signal. The adaptive filter typically uses an adaptive Recursive Least Squares (RLS), Least Mean Squares (LMS) or Minimum Mean-Square Error (MMSE) estimation as an algorithm.
Modern communication systems typically transmit a preamble (i.e. a known waveform section) which demodulators can use to achieve waveform synchronization. In addition, the preamble may also contain information transmitted in a very robust fashion used to indicate the waveform parameters used for the data portion of transmission that follows this initial preamble (i.e. modulation type (2-PSK, 4-PSK, 8-PSK, 16-QAM, etc), burst length, type of forward error correction, etc). When a system incorporates waveform information in its preamble it is referred to as an autobaud system. When another mechanism is used to convey this information, such as a control channel or a separate transmission providing this information, the preamble is used only for synchronization. It may be advantageous to a communication system to feedback demodulator state (preamble search state, preamble state, and data state) and demodulator information (modulation type, etc) to adaptive filter to more effectively deal with narrowband interference while reducing the effects of adaptive filter on the data portion of waveform.
There are many design tradeoffs when using adaptive filters in modern wideband UHF/VHF tactical radios such as number of filter taps, speed of adaptation, etc. In addition, many platform (i.e. radio hardware) constraints such as size, weight, power and relatively small Field Programmable Gate Array (FPGA) usage are imposed on the adaptive filter design. The proper tradeoff in the adaptive filter design is necessary so that more than one interferer can be handled by adaptive filter (typically three to four is desired) while still meeting platform constraints. Examples of interferers in the HF/VHF/UHF band are analog frequency modulation (FM) voice, frequency shift keying (FSK) signals (i.e. 16 kbps FSK), tone signals or carriers.
Some adaptive filters incorporate spectral based techniques that use a Fast Fourier Transform (FFT), thus, making them too complex for some radio implementations. Adaptive filters such as Finite Impulse Response (FIR) filters, or Infinite Impulse Response (IIR) filters exist and have been found to work well at lower bit rates and bandwidths. Both types of filters, however, have some drawbacks, but with improvements, should provide important design enhancements for adaptive filters used in complex communications systems.